Pressure difference sensors are applied in industrial measurements technology for measuring pressure differences resulting from a pressure difference between a first and a second pressure acting on the pressure difference sensor.
Applied as pressure difference sensors are pressure difference sensors referred to e.g. as semiconductor sensors or sensor chips, which can be produced cost effectively on an undivided wafer by applying processes known from semiconductor technology.
These pressure difference sensors usually have a measuring membrane arranged between two platforms. Pressure chambers are enclosed In each of the two platforms, under the measuring membrane. In measurement operation, the first side of the measuring membrane is supplied with the first pressure via an opening in the first platform and the second side of the measuring membrane is supplied with the second pressure via an opening in the second platform.
In the case of capacitive pressure difference sensors, the deflection of the measuring membrane resulting from the difference between the two pressures is registered by means of a capacitive, electromechanical transducer and converted into an electrical signal reflecting the pressure difference to be measured. Semiconductor sensors have, regularly, measuring membranes of silicon, which, due to its conductivity, can be directly applied as an electrode of the capacitive transducer. Additionally, capacitive transducers comprise at least one rigid counterelectrode integrated into one of the two platforms and electrically insulated relative to the measuring membrane for forming together with the measuring membrane serving as electrode a capacitor. The capacitances of these capacitors depend on the deflection of the measuring membrane, which, in turn, depends on the pressure difference to be measured.
Fundamentally, it would be possible to equip pressure difference sensors with one-piece platforms forming the counterelectrodes, between which platforms there is arranged a silicon disk serving as measuring membrane and at the same time as electrode. For this, insulating layers are provided between the silicon disk and each of the two counterelectrodes. Via these insulating layers, an outer edge of the silicon disk is connected with the outer edges of the respective counterelectrodes for enclosing the respective pressure chambers.
Use of such pressure difference sensors is discouraged in German patent, DE 38 27 138 A1, since in the case of these pressure difference sensors, there is the problem, described in detail in German patent, DE 38 27 138 A1, that each of the two capacitors formed by the silicon disk and one of the two one-piece platforms is composed of an inner capacitor portion and an outer capacitor portion externally surrounding the inner capacitor portion. The inner capacitor portion is located in the region of the pressure difference sensor, in which the central region of the silicon disk experiences the deflection dependent on the pressure difference. The outer capacitor portion is located in the region of the pressure difference sensor, in which the outer edge of the silicon disk surrounding the central region is arranged between the insulating layers.
The capacitance C1, C2 of each of the two capacitors corresponds to the sum of the capacitances C1a, C1b, respectively C2a, C2b, of the two capacitor portions, of which they are composed. In such case, however, only the capacitance C1a, respectively C2a, of the inner capacitor portion has the pressure difference dependence metrologically to be registered. The has the result that the capacitance change ΔC1a, respectively ΔC2a metrologically to be registered dependent on the pressure difference dependent deflection of the inner region of the silicon disk is small in comparison to the measured capacitance C1=C1a+C1b, respectively C2=C2a+C2b, given by the sum of the capacitances. Correspondingly, the accuracy of measurement achievable thereby is very small.
Also, the differential change f of the two capacitances C1, C2 described in German patent, DE 38 27 138 A1 and utilized in pressure measuring technology for ascertaining the pressure difference, as determined based on the ratio of the difference C1−C2 of the two capacitances to their sum C1+C2, i.e. f=(C1−C2)/C1+C2), does not have the desired linear dependence on the pressure difference to be measured.
These disadvantages are especially significant in the case of pressure difference sensors with square footprint, since the electrode areas operative for the capacitances C1b, C2bof the outer capacitor portions are especially large in the case of silicon disk and counterelectrodes with corresponding square footprints.
Semiconductor sensors manufacturable in the undivided wafer have, however, regularly, square footprint surfaces, since square footprint surfaces enable the pressure difference sensors manufactured in the undivided wafer to be separated into single chips by sawing along straight lines.
This problem is solved in the state of the art in the manner described e.g.in German patent, DE 38 27 138 A1 and in DE 103 93 943 B3 by applying special platforms on both sides of the silicon disk forming the measuring membrane. These platforms are constructed of three layers arranged on top of one another and have, in each case, a conductive layer facing the silicon disk and a conductive layer facing away from the silicon disk. The conductive layers are separated from one another by an insulating layer arranged between the two conductive layers. In the conductive layer facing the silicon disk, there is provided, in each case, at least one annular moat leading to the insulating layer. The annular moat divides the layer into a region surrounded by the moat and serving as counterelectrode, and an outer region externally surrounding the moat and supporting the silicon disk. In such case, the inner region is structured in such a manner that it is spaced from the silicon disk.
The manufacture of such pressure difference sensors is, however, comparatively complicated, since each platform must be constructed of a number of layers, and the individual layers must be correspondingly structured and bonded with one another. Moreover, the counterelectrodes enclosed in the platforms must be electrically contacted through the outer layer and the insulating layer of the respective platforms.